1. Field of the Invention
The present invention relates generally to a molding apparatus. More particularly, the invention relates to a molding apparatus for encapsulating semiconductor device packages, which can produce semiconductor packages free from molding defects.
2. Description of the Related Art
In a process for fabricating semiconductor device packages, a semiconductor device attached to a lead frame is encapsulated with a molding compound in order to provide a protection of the semiconductor device from external environments. The encapsulation is called as molding process.
Although the active surface of semiconductor device is coated with an oxide layer and an organic layer, the mechanical and chemical strength of these layers are not sufficient so that the encapsulation is needed to provide a proper protection of the active surface of semiconductor device, as well as to protect the electrical connections between the semiconductor device and the lead frame.
In general, transfer molding apparatuses using epoxy molding compounds are commonly employed for the molding process. The transfer molding process is more widely used than other encapsulation methods for encapsulating semiconductor devices because of its advantages such as good process stability and ease of mass-production. The conventional transfer molding apparatus will be explained below.
FIG. 1a to FIG. 1c show consecutive steps of a molding process using a conventional transfer molding apparatus. FIG. 1a shows the state before introducing molding compound into the runners and the cavities of the mold body, FIG. 1b shows the early state of the introduction of molding compound, and FIG. 1c shows the state after completing the introduction of molding compound into the mold body.
With reference to FIG. 1a, the conventional transfer molding apparatus 50 has a mold body 60 having an upper mold die 61 and a lower mold die 62. A semiconductor device 41 attached to a lead frame 40 is placed in a cavity 67 between the upper mold die 61 and lower mold die 62, and molding compound tablets 45 are loaded into a pot 55. The tablets 45 are heated and transformed to fluid molding compound 46. Then, as shown in FIG. 1b and FIG. 1c, this fluid molding compound 46 flows into the runners 63 and the cavity 67, when a plunger 57 moves upward and pushes the fluid molding compound 46. And, the fluid molding compound 46 is solidified by curing of the fluid molding compound 46, and provides a protection of the semiconductor device 41 from external environments.
The transfer molding apparatus 50 has a cull-block 70 in the upper mold die 61 to make the movement of the fluid resin 46 easier. The cull block 70 is located to face the pot 55 and the upper surface of the plunger 57.
The molding compound 45 is in the form of tablet when it is loaded into the pot 55 of the transfer molding apparatus 50. After being loaded into the pot 55, the molding compound tablet 45 is heated to a predetermined elevated temperature by supporting plates 52 that include a heater 59, and transformed to fluid molding compound 46. During this transformation, air contained in the tablet 45 remains in the fluid molding compound 46. Supporting plates 52 have two parts, upper plate 53 and lower plate 54.
When the fluid molding compound 46 flows into and fills the cavity 67 through the runner 63 by the movement of the plunger 57, the air may remain as air traps 48 at the interface between the surface of the cavity 67 and the fluid molding compound 46, or as voids 49 within the fluid molding compound 46. Although air vents 68 are made to provide an exit for the air in the fluid molding compound 46, sometimes the air traps 48 and the void are not completely eliminated.
After the fluid molding compound 46 is cured, the air traps 48 may result in the packages with defective surfaces, and the void 49 may cause package cracking later in surface mounting.
There is another significant source for the air included in fluid molding compound 46. The air within the space 78 between the cull-block 70 and the pot 55 is difficult to eliminate completely through the air vent 68 before the fluid molding compound 46 flows into the runner 63. In general, the fluid molding compound 46 occupies the runner 63 before the air in the space 78 are completely removed, leaving air gaps 47 on the surface of the cull-block 70, as shown in FIG. 1b. As the plunger 57 keeps on moving upward, the air gaps 47 are introduced belatedly into the runner 63 and the cavity 67, and produce air traps 48 and voids 49. That is, unless the cull-block 70 is designed properly, the air gaps 47 cannot be removed completely, resulting in a frequent occurrence of void formation or incomplete molding.
Moreover, a delayed elimination of the air in the fluid molding compound 46 through the air vent 68 may cause a clogging of the air vent 68 (known as `film phenomenon`). This clogging of the air vent 68 may interfere further elimination of the air through the air vent 68, and cause a severe failure of the molding process.